Java wysigers
Wysigers
Teen hierdie tyd is jy redelik bekend met die public
sleutelwoord wat in byna al ons voorbeelde voorkom:
public class Main
Die public
sleutelwoord is 'n toegangswysiger , wat beteken dat dit gebruik word om die toegangsvlak vir klasse, eienskappe, metodes en konstrukteurs te stel.
Ons verdeel wysigers in twee groepe:
- Toegangswysigers - beheer die toegangsvlak
- Nie-toegangswysigers - beheer nie toegangsvlak nie, maar bied ander funksionaliteit
Toegang wysigers
Vir klasse kan jy een public
of verstek gebruik :
Modifier | Description | Try it |
---|---|---|
public |
The class is accessible by any other class | |
default | The class is only accessible by classes in the same package. This is used when you don't specify a modifier. You will learn more about packages in the Packages chapter |
Vir eienskappe, metodes en konstruktors kan jy die een van die volgende gebruik:
Modifier | Description | Try it |
---|---|---|
public |
The code is accessible for all classes | |
private |
The code is only accessible within the declared class | |
default | The code is only accessible in the same package. This is used when you don't specify a modifier. You will learn more about packages in the Packages chapter | |
protected |
The code is accessible in the same package and subclasses. You will learn more about subclasses and superclasses in the Inheritance chapter |
Nie-toegang wysigers
Vir klasse , kan jy óf final
of abstract
:
Modifier | Description | Try it |
---|---|---|
final |
The class cannot be inherited by other classes (You will learn more about inheritance in the Inheritance chapter) | |
abstract |
The class cannot be used to create objects (To access an abstract class, it must be inherited from another class. You will learn more about inheritance and abstraction in the Inheritance and Abstraction chapters) |
Vir eienskappe en metodes kan jy die een van die volgende gebruik:
Modifier | Description |
---|---|
final |
Attributes and methods cannot be overridden/modified |
static |
Attributes and methods belongs to the class, rather than an object |
abstract |
Can only be used in an abstract class, and can only be used on methods. The method does not have a body, for example abstract void run();. The body is provided by the subclass (inherited from). You will learn more about inheritance and abstraction in the Inheritance and Abstraction chapters |
transient |
Attributes and methods are skipped when serializing the object containing them |
synchronized |
Methods can only be accessed by one thread at a time |
volatile |
The value of an attribute is not cached thread-locally, and is always read from the "main memory" |
Finale
As jy nie die vermoë wil hê om bestaande kenmerkwaardes te ignoreer nie, verklaar kenmerke as final
:
Voorbeeld
public class Main {
final int x = 10;
final double PI = 3.14;
public static void main(String[] args) {
Main myObj = new Main();
myObj.x = 50; // will generate an error: cannot assign a value to a final variable
myObj.PI = 25; // will generate an error: cannot assign a value to a final variable
System.out.println(myObj.x);
}
}
Staties
'n static
Metode beteken dat toegang daartoe verkry kan word sonder om 'n objek van die klas te skep, anders as public
:
Voorbeeld
'n Voorbeeld om die verskille tussen static
en public
metodes te demonstreer:
public class Main {
// Static method
static void myStaticMethod() {
System.out.println("Static methods can be called without creating objects");
}
// Public method
public void myPublicMethod() {
System.out.println("Public methods must be called by creating objects");
}
// Main method
public static void main(String[ ] args) {
myStaticMethod(); // Call the static method
// myPublicMethod(); This would output an error
Main myObj = new Main(); // Create an object of Main
myObj.myPublicMethod(); // Call the public method
}
}
Abstrak
'n abstract
Metode behoort aan 'n abstract
klas, en dit het nie 'n liggaam nie. Die liggaam word verskaf deur die subklas:
Voorbeeld
// Code from filename: Main.java
// abstract class
abstract class Main {
public String fname = "John";
public int age = 24;
public abstract void study(); // abstract method
}
// Subclass (inherit from Main)
class Student extends Main {
public int graduationYear = 2018;
public void study() { // the body of the abstract method is provided here
System.out.println("Studying all day long");
}
}
// End code from filename: Main.java
// Code from filename: Second.java
class Second {
public static void main(String[] args) {
// create an object of the Student class (which inherits attributes and methods from Main)
Student myObj = new Student();
System.out.println("Name: " + myObj.fname);
System.out.println("Age: " + myObj.age);
System.out.println("Graduation Year: " + myObj.graduationYear);
myObj.study(); // call abstract method
}
}